Extending turbine lifetime brings down CoE

Wind turbines are still often developed for a 20-year design life. However, a number of recent initiatives indicate a gradual shift to a 25-year and even longer operating period. And with turbines in the 4.5-7.5MW class becoming more common, it is hard to imagine these giants being knocked down after operating only for two decades.

The single largest benefit for owners and operators from lifetime extension is driving down the lifecycle-based costs of energy (CoE). A wider overall benefit to societies as a whole is enhancing the sustainability of renewable products and projects.

Wind-farm sustainability

All wind turbines have a number of main infrastructural components, such as transformers and medium-voltage copper ground cables for electricity transport, that can last 50 years or more. The same applies to other durable components inside wind turbines, provided they are not exposed to lifetime-shortening factors, such as continuous vibration, major temperature fluctuations, silt air and humidity.

In addition, the latest 3-7.5MW onshore turbines are increasingly put on concrete or concrete-steel hybrid towers. These structures can again last 50 years or more due to only minor material fatigue-related impact on design life compared to steel sections performing similar tasks. The same design life range can be expected for concrete foundations.

These components represent a big share of total capital investment. Spreading their payback over a longer period contributes towards lowering CoE, while extending the active use phase of the available infrastructure improves sustainability.

For offshore applications, a wind turbine represents only about 35% of total capital investment. This relatively low proportion is due to the high installation cost of heavy and expensive support and top structures and long electricity-distribution and transport cables to shore.

Mixed results

Initiatives to extend turbine lifetime are not new but have in the past produced mixed results. In the mid-1990s, former US wind-turbine supplier Zond displayed a scale model of its then-new 750kW Z-750 model at Germany’s Hanover international industry fair. Among its distinct features was a huge integrated drive train and a doubly fed induction generator — a wind technology novelty. The company claimed it to be the world’s only turbine with a GL-certificate for a 30-year design life. Unfortunately, soon after its introduction on to the market, the Z-750 developed significant problems with key components such as gearboxes and generators. These required a complete redesign.

However, there are examples of kilowatt-class turbine models built from the late 1970s until the early 1990s that have successfully operated over 20 years without requiring substantial replacement of main components.

The early wind industry days were characterised by substantial knowledge gaps, limited know-how and insufficient track-record experience. At the same time, constant market pressures to rapidly increase turbines’ rated capacity made it difficult for designers to gain larger-series experience needed to develop more reliable turbines while adding to technical and operational risk as well as costs. Owners and developers, meanwhile, often opted to remove older turbines well before they had reached their technical lifespan, replacing these with larger, more powerful new models.

Utility turbines

With the emergence of utility-grade 1.5-2MW and above turbines from the mid-1990s onwards, pressure to upscale has eased somewhat, and these power ratings are in fact widely expected to stay. Such megawatt-scale turbines are now being offered in a wide variety of makes and models. Generally, performance has been rather good, but there have been exceptions. Over the recent few years, gearboxes have attracted particular criticism, often being blamed as the single main cause of turbine failure.

Several turbine series, including the Vestas 2MW, Nordex 2.5MW, GE 1.5MW, Gamesa 2MW and Enercon 1.5-2MW have now been on the market for a decade or longer, typically in multiple versions. The Siemens 2.3MW turbine entered the market in 2002, and three new, larger rotor diameters have been introduced since — from the initial 82.4 metre to 93 metre, 101 metre and 108 metre.

This strategy is now common across the industry. It aims to boost yield performance while driving down CoE and extending turbine platform life. Manufacturing large numbers of similar turbines encourages production efficiencies and standardisation of components while lowering manufacturing and procurement costs.

Another major leap forward can be made in terms of long-term turbine upkeep including the offering of mid-life upgrades. Enercon offers a long-term service package, which guarantees the owner or operator 97% technical availability in return for an annual fee that is in proportion to and a reflection of generated energy yield. An integral part of this contract is that Enercon can implement hardware and/or software upgrades if these are considered beneficial for enhanced performance or availability. This in fact represents a mutual benefit for both supplier and customer.

Repowering

In several main wind markets large-scale repowering of sites occupied by ten- to 20-year-old kilowatt-size turbines with larger modern equivalents was expected to become a main trend. One recent Dutch example involved replacing three 225kW and 250kW turbines with two 2.3MW turbines, increasing annual energy production by a factor of eight. A potential problem is that plans for repowering are in practice often hampered by permitting constraints, such as installation height restrictions and/or limited grid capacity.

Growing business

Spanish manufacturer Gamesa set up a specialist business unit in 2010 offering dedicated, large-component reconditioning and lifetime extension services. "One of our learning examples is the nuclear industry, where the lifetime extension concept has been common practice for decades," Christian Jourdain, marketing manager for the service business, said at the recent European Wind Energy Association conference in Denmark. "These projects typically commence with a technology condition scan after around 15 years of operation, serving as the basis for a production improvement plan of the additional years."

The large-component reconditioning unit focuses on maximising wind turbine performance while reducing turbine downtime and operation-and-maintenance costs. When a large component — such as a gearbox, generator or blade — fails prematurely, the first action is always disassembly and diagnosis of the root cause, with a detailed report sent to the client.

Gamesa’s clients then have a choice between "standard" and "premium" repair. The standard option includes the use of state-of-the-art remanufactured components whose specifications are technically identical to those of original components. Afterwards, the turbine returns to operation and can be expected to continue operating with unchanged "new" component useful life expectancy and similar statistical failure chances.

In the premium option, Gamesa technicians apply root-cause analysis to enhancements necessary to bring specific turbine components to the latest technological level. "Each component is further subjected to full-load bench testing up to the operational limits," Jourdain explained. "Because the enhancements significantly reduce a component’s failure rate, this in turn has substantial positive impact upon future failure rates based on statistical occurrence. After component reconditioning incorporating design enhancements, ‘new useful life’ this time in fact jumps over the former ‘theoretical useful life’ curvature, thus extending both time in service and remaining useful life."

Extending the operational lifetime of main components has been the first step of the turbine life extension programme initiated by Gamesa three years ago. Jourdain stresses that Gamesa has its own gearbox, generator and blade-manufacturing divisions, each with its own design teams and in-depth expertise.

Third-party suppliers

To further reduce turbine downtime in the event of a major failure, Gamesa keeps significant stock of large components including third-party supplier parts. Included on the comprehensive list are reconditioned gearboxes, generators and various blade types for other manufacturers’ wind turbines. Modern turbines usually recover their cumulative energy inputs within 3-8 months of electricity generation. This is called the energy payback period. An interlinked, lifecycle-based energy output-input ratio — or sustainability performance number — is known as the "harvest factor".

A 20-year turbine lifetime with a six-month energy payback period results in a harvest factor of 20 x 12 / 6 = 40. Finally, expanding turbine operation to 25-30 years substantially contributes to harvest factors, provided upkeep costs and turbine performance remain in line with predetermined industry expectations. Combined with improved material and infrastructure utilisation, overall lifecycle enhancement benefits will further drive down CoE. This is a clear win-win situation that fits seamlessly into the key expanding role modern wind power can — and does already — offer the world.

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